Stents, grafts and a variety of other endoprostheses are well known and used in interventional procedures, such as for treating aneurysms, for lining or repairing vessel walls, for filtering or controlling fluid flow, and for expanding or scaffolding occluded or collapsed vessels. Such endoprostheses can be delivered and used in virtually any accessible body lumen of a human or animal, and can be deployed by any of a variety of recognized means. One recognized indication of endoprostheses, such as stents, is for the treatment of atherosclerotic stenosis in blood vessels. For example, after a patient undergoes a percutaneous transluminal coronary angioplasty or similar interventional procedure, an endoprosthesis, such as a stent, is often deployed at the treatment site to improve the results of the medical procedure and to reduce the likelihood of restenosis. The endoprosthesis is configured to scaffold or support the treated blood vessel; if desired, the endoprosthesis can also be loaded with beneficial agent so as to act as a delivery platform to reduce restenosis or the like.
The endoprosthesis is typically delivered by a catheter delivery system to a desired location or deployment site inside a body lumen of a vessel or other tubular organ. To facilitate such delivery, the endoprosthesis must be capable of having a particularly small cross profile to access deployment sites within small diameter vessels. Additionally, the intended deployment site may be difficult to access by a physician and often involves traversing the delivery system through the tortuous pathway of the anatomy. It therefore is desirable to provide the endoprosthesis with a sufficient degree of longitudinal flexibility during delivery to allow advancement through the anatomy to the deployed site.
Generally endoprosthesis' are constructed of multiple rings that are connected either through a connection section or a connection element, wherein the number of connection sections or elements as well as the thickness of the struts that comprise the rings control the flexibility of the endoprosthesis. Although it is not specifically known how much vessel restenosis can be attributed to stent rigidity, it is know that a reasonably stiff stent may injure the vessel during motion (i.e. pulsatile heart movement). Therefore, it is desirable to produce an endoprosthesis, which has good stiffness properties for deployment within a vessel and wherein the stiffness properties of the endoprosthesis can be changed after deployment within a vessel.
Once deployed, the endoprosthesis should be capable of satisfying a variety of performance characteristics. The endoprosthesis should have sufficient rigidity or outer bias when deployed to perform its intended function, such as opening a lumen or supporting a vessel wall. Similarly, the endoprosthesis should have suitable flexibility along its length when deployed so as not to kink or straighten when deployed in a curved vessel. It also may be desirable to vary the rigidity or flexibility of the endoprosthesis along its length, depending upon the intended use. Additionally, it may be desirable for the endoprosthesis to provide substantially uniform or otherwise controlled coverage, e.g., as determined by the ratio of the outer surface of the endoprosthesis to the total surface of the vessel wall along a given length. For example, increased coverage may be desired for increased scaffolding, whereas decreased coverage may be desired for side access to branch vessels. Control of the cross profile and length of the endoprosthesis upon deployment also is desirable, at least for certain indications.
Numerous designs and constructions of various endoprosthesis embodiments have been developed to address one or more of the performance characteristics summarized above. For example, a variety of stent designs are disclosed in the following patents: U.S. Pat. No. 4,580,568 to Gianturco; U.S. Pat. No. 5,102,417 to Palmaz; U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 5,133,732 to Wiktor; U.S. Pat. No. 5,292,331 to Boneau; U.S. Pat. No. 5,514,154 to Lau et al.; U.S. Pat. No. 5,569,295 to Lam; U.S. Pat. No. 5,707,386 to Schnepp-Pesch et al.; U.S. Patent 5,733,303 to Israel et al.; U.S. Pat. No. 5,755,771 to Penn et al.; U.S. Pat. No. 5,776,161 to Globerman; U.S. Pat. No. 5,895,406 to Gray et al.; U.S. Pat. No. 6,033,434 to Borghi; U.S. Pat. No. 6,099,561 to Alt; U.S. Pat. No. 6,106,548 to Roubin et al.; U.S. Pat. No. 6,113,627 to Jang; U.S. Pat. No. 6,132,460 to Thompson; and U.S. Pat. No. 6,331,189 to Wolinsky; each of which is incorporated herein by reference.
An additional problem with existing endoprosthesis designs is the difficulty in properly placing the endoprosthesis within a vessel prior to deployment of the endoprosthesis. Current endoprosthesis designs have thinner struts that utilize less radiopaque material and therefore do not appear as well under fluoroscopy. An attempt to address the reduced radiopacity is to include at least one marker band disposed on the delivery device, wherein the marker band may be utilized to indicate an end of the endoprosthesis device or any length there along. Other methods of increasing the radiopacity of an endoprosthesis include the addition of radiopaque markers either disposed upon a surface of the endoprosthesis or within a retaining member associated with endoprosthesis. A shortcoming of present designs is that many are very difficult to manufacture and therefore lead to increased costs. Also, due to size limitations of the radiopaque material used, the markers do not provide sufficient visibility for precise placement.
Another limitation of current endoprosthesis designs is their unsuitability for materials with high elastic limits such as bioabsorbable polymers. The expansion of endoprosthesis devices such as stents generally relies on the plastic deformation of the stent material and typical stent designs do not undergo enough strain during expansion to plastically deform bioabsorbable polymers. This can result in excessive recoil of the stent and sub-optimal apposition of the stent against the vessel wall. Therefore, it is also desirable to provide a stent design that enables expansion of the stent to a greater diameter without plastically deforming the stent material.
Although the various designs for endoprostheses that have been developed to date may address one or more of the desired performance characteristics, there a remains need for a more versatile design for an endoprosthesis that allows improvement of one or more performance characteristics without sacrificing the remaining characteristics.